To do this, you'll want to log in to BoxLock Control, and find the lock you'd like to open. Once done, you can click on that lock to arrive at the Lock Details page, shown below.
Here there's a button to the right of the "Add Barcode" button of the screen that will read "Push to Unlock". Once you've click it you'll be presented with a dialogue box that contains a countdown.
As long as the countdown is active, the person with the lock can press the button on it, and the lock will open. If they aren't able to open the lock in time, they can always click "Try Again" and the countdown will restart.